The exhaust gas discharged from an internal combustion engine in, e.g., an automobile, contains pollutant components such as carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxides (NOx). In order to remove these pollutant components from the exhaust gas, an exhaust gas purification apparatus equipped with an exhaust gas purification catalyst is disposed in the exhaust path of the internal combustion engine. A three-way catalyst, which simultaneously performs oxidation of the CO and HC and reduction of the NOx, is preferably used for this exhaust gas purification catalyst. A widely known three-way catalyst here generally has a precious metal catalyst, e.g., platinum (Pt), rhodium (Rh), palladium (Pd) supported on a porous support made of a metal oxide such as alumina (Al2O3), and exhibits a high catalytic capacity in particular for the exhaust gas produced when an air-fuel mixture in the neighborhood of the theoretical air/fuel ratio (stoichiometry: A/F=14.7) is fed to the internal combustion engine.
However, it is difficult to continuously maintain the air/fuel ratio of the air-fuel mixture actually fed to an internal combustion engine in the neighborhood of stoichiometry, and depending on, for example, the running conditions for the automobile, the air/fuel ratio of the air-fuel mixture may assume a fuel excess condition (rich: A/F<14.7) or an oxygen excess condition (lean: A/F>14.7). As a consequence, in recent years the support has included an inorganic material that has an oxygen storage capacity (OSC), i.e., an OSC material. This OSC material absorbs and stores the oxygen in the exhaust gas when the air-fuel mixture has gone lean (this exhaust gas is referred to below as a “lean exhaust gas”) and thereby facilitates reduction of the NOx in the exhaust gas by making the exhaust gas into a reducing atmosphere. On the other hand, the OSC material releases the stored oxygen when an exhaust gas is supplied from an air-fuel mixture that has gone rich (this exhaust gas is referred to below as a “rich exhaust gas”), thereby facilitating oxidation of the CO and HC in the exhaust gas by making the exhaust gas into an oxidizing atmosphere.
Feedback control (F/B control) is also widely used in exhaust gas purification apparatuses. In this F/B control, the oxygen concentration upstream from the exhaust gas purification catalyst (the upstream O2 concentration) is first detected and a first control target value is established based on this upstream O2 concentration and a prescribed target air/fuel ratio (main F/B control). The oxygen concentration downstream from the exhaust gas purification catalyst (the downstream O2 concentration) is also detected and a second control target value is established by correcting, based on this downstream O2 concentration, the first control target value that has been established by the main F/B control (sub-F/B control). In addition, by adjusting the air/fuel ratio of the air-fuel mixture based on this second control target value, an air-fuel mixture can be fed to the internal combustion engine that reflects the air/fuel ratio of the current air-fuel mixture and the status of the exhaust gas purification catalyst. By adjusting the air/fuel ratio of the air-fuel mixture to an appropriate condition, such F/B control contributes to improving fuel consumption and improving the efficiency of purification of the pollutant components in the exhaust gas.
However, a certain time lag (control lag) can be produced in air/fuel ratio control of the air-fuel mixture by the F/B control described in the preceding, and exhaust gas from an unsuitable air/fuel ratio will continue to be fed to the exhaust gas purification catalyst in the time interval that this control lag produces. In this case, the exhaust gas purification function due to the exhaust gas purification catalyst does not operate properly and emissions are then produced in which pollutants in the exhaust gas are discharged to the outside.
Considered broadly, this control lag is composed of a “transport lag” and a “response lag”. The “transport lag” refers to the time lag after the second control target value for the air-fuel mixture has been set by F/B control until the an air-fuel mixture that reflects this second control target value is combusted in the internal combustion engine and converted to exhaust gas and reaches the exhaust gas purification catalyst. The “response lag”, on the other hand, refers to the time lag until an exhaust gas comes into contact with the O2 sensors for detecting the upstream O2 concentration and the downstream O2 concentration and the second control target value based on the output of these O2 sensors is established.
The response lag, which is one factor in the control lag as described above, is produced by, for example, a decline in the responsiveness of the downstream O2 sensor that detects the downstream O2 concentration. A typical oxygen sensor can exhibit a high responsiveness when oxygen is present to the extent that oxygen is at the periphery of the sensor element, but the responsiveness deteriorates when oxygen is not present at the periphery of the sensor element. Due to this, when excess rich exhaust gas is continuously fed to the downstream O2 sensor, a state is reached in which there is almost no oxygen at the periphery of the sensor element, and the responsiveness is then diminished when a lean exhaust gas subsequently begins to be supplied. Subsequent to this, even when the target air/fuel ratio is adjusted to the lean side and an exhaust gas having a high oxygen concentration is supplied to the downstream O2 sensor, a correct value for the existing downstream O2 concentration cannot be detected; the correction for the second control target value by the previously described sub-F/B control is then not appropriately performed for some period of time; and the aforementioned response lag is produced.
A control apparatus for suppressing the control lag in the aforementioned F/B control is disclosed in Patent Literature 1. This control apparatus is provided with an exhaust gas purification catalyst that has a lower oxygen storage rate on the side facing the downstream O2 sensor (the downstream region of the exhaust gas purification catalyst has a slower oxygen storage rate than the upstream region) than on the side facing the upstream O2 sensor. Specifically, in the control apparatus described in Patent Literature 1, the oxygen storage rate on the outlet side is slowed down by reducing the amount of precious metal supported in the downstream region of the exhaust gas purification catalyst and/or by reducing the Zr compositional ratio in the OSC material in the downstream region. A control apparatus with this structure can stop the feed of excess rich exhaust gas to the downstream O2 sensor because, even when the amount of oxygen storage by the OSC material in the upstream region declines, small amounts of oxygen continue to be released from the downstream region OSC material with its slower oxygen storage rate. As a consequence, the response lag due to the impaired responsiveness of the downstream O2 sensor as described above can be suppressed. In addition, other art related to exhaust gas purification catalysts is disclosed in Patent Literature 2 and 3.